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The Finish Line: Rust

But what happens if discoloration or a streak occurs on a large EIFS wall area-a ‘blotch’ where there shouldn’t be one. It draws attention to itself, and results in angry phone calls.

One source of blotches is ‘rust,’ iron and carbon steel.

The rust problem is not an EIFS problem per se, but is related to the textured coatings that are on the outside surface of EIFS-the EIFS “finish”-and to related wall components. This month’s column is about rust, in its various forms, and how to deal with it.

What is Rust?

Rust comes from iron, an element found in raw form. Steel is made using iron as its main ingredient. Iron is rarely used anymore in construction (by the way, the Eiffel Tower is iron, not steel), but steel obviously is used all over the place on buildings. Raw steel is virtually never used, as it will rust immediately, but is coated with something, such as paint or zinc (galvanized). It’s not that simple though. Stainless steel has iron in it too, but it doesn’t rust. How come? Like aluminum, stainless steel naturally produces a tight, minute surface condition when in the presence of air. This film keeps it from corroding or rusting in most environments. There are many types of aluminum and stainless steel, and some are more resistant to corrosion than others.

Iron, when combined with water, produces ferrous oxide, which can become water-born and cause ugly streaks on walls. As the water reacts chemically with the iron, it expands and becomes loose on the surface. The surface iron then flows away in the water, exposing the iron again to water. The process goes on until the iron or steel “rusts through”.

Types of Rust

Rust with EIFS is a problem with the finish and does not affect the rest of the EIFS. It is an aesthetic issue and causes no functional problems. With EIFS, there are a couple types of rust:

Internal rust, which includes microscopic iron in the finish and iron particles within the finish.

Embedded surface rust, which is within the surface of the finish.

Surface rust, which is on the surface of the finish.

Surface Rust

This is easy to deal with, as it is simply lying on the surface; it is a topical problem. It can be removed with ferrous stain removing liquid cleaners. There are specialty cleaners for EIFS that work better than the general type you find in hardware stores. Be careful what kind you use, as some are quite powerful chemically, and can bleach the color of the finish or attack the EIFS foam layer.

Embedded surface rust is harder to deal with. Here’s how it happens: Rust stains form when iron reacts with water and the iron gets into the water as a sort of “tint” since there is water near the rust source anyway (say, a bolt going through the EIFS, and rain or dew on the wall), and the water is present already. The water slightly softens the finish and the iron-bearing water gets into the surface. When the finish dries, the iron stain gets locked into the surface of the finish. Sometimes this can be removed with a ferrous stain remover, but often it’s easier to paint over the rusty area. It’s important to get rid of the iron source; replace the rusty bolt with a stainless steel one, etc. To get a good match, you can take a color sample to the paint store and have them make a small batch of paint for you. Or, if you happen to have some left over EIFS finish, you can run it through cheese cloth (straining the aggregate out of the finish) and make a “paint” out of it, and then brush or roller it on.

Embedded Rust

Sometimes there is iron within the finish. This can take the form of tiny chunks of iron, such as weld spatter or steel chips from a chop saw, that got thrown up against the finish before it has dried, and got stuck within the finish.

More commonly, the aggregate used to make the finish has tiny iron particles either loose in it, or stuck onto the side of an aggregate particle. This brings up the subject of aggregate types.

Aggregate Types

Many EIFS use natural sand as the aggregate. This is sand that comes from sand deposits, such as ancient river beds (ocean beach sand is not used, as it contains all sorts of odd other things that would need to be cleaned out). The sand is merely dug up, washed, graded for size, and used; it is not normally crushed. Sand varies considerably in its quality, from pure to dirty. This is why many EIFS producers have their factories near major deposits of clean natural sand. Also, sometimes the layers within a given sand deposit have varying degrees of iron. For this reason some EIFS producers have the sand batches screened for iron by measuring how many iron particles there are per million grains of sand. Some EIFS producers also have magnets in their product line that attempt to pull iron bearing sand particles from the sand as the finish coating is being made.

Some EIFS use crushed marble as the aggregate. Marble isn’t normally as susceptible to iron contamination as sand. Some producers also use man-made grit particles that are a byproduct of other industrial processes and hence are more pure.

Digging Out The Offender

It’s surprising how an iron particle the size of a pinhead can produce a stain several feet long. If you have the patience, the stain will eventually go away on its own, leaving a less objectionable whitish streak. However, often building owners want iron streaks fixed now. This means getting rid of the surface stain (see above) but more importantly getting rid of the source-the tiny rust particle.

You can go to the top end of the rust stain and often see the offending iron particle. A nut picker is good for digging out the particle. Painting over the particle doesn’t usually work since the moisture in the air and on the wall will make the particle continue to rust, causing the stain to bloom through the paint. Once the particle is gone, you can remove the surface rust with a stain remover liquid, or paint over the surface rust.

Milder Rust Problems

Sometimes iron can cause ‘rust’ problems that are not so much a distinct stain but rather an off-color. This can be caused by such things as:

• Iron contaminated water, which you should be able to notice when adding water to the finish

• Using iron-contaminated water to clean tools

• Using tools and equipment that are rusty, such as trowels, mixing paddles, and buckets.

This tiny amount of iron can cause the color of the finish to look “off.” Since it is throughout the finish, the way to fix it is to paint over it.

The problem of rust stains is not unique to EIFS. Coatings of similar types that are used on concrete, direct-applied finish systems, and stucco can have the same problems. This problem is most common in warm, humid, salty environments such as the U.S. Gulf Coast where the iron starts to rust quickly, sometimes before the building is even completed.

You can see that rust problems can occur as part of the manufacturing process, as well as in the field. The worst case I ever saw was due to a disgruntled plasterer who dumped a small handful of iron filings in each bucket of finish. The effect was immediate and all too obvious, and it did not take long to figure out what had happened.

There was a period, over a decade ago, when there was a rash of rust stain problems with EIFS. Since some EIFS producers get their aggregate from the same suppliers as their competitors, it became an EIFS industry problem. After much detective work, the problem was identified-iron-bearing sand-and measures were taken to eliminate the iron.

These days rust stain problems due to the EIFS product itself, or its application, are rare. Most rust problems are due to adjacent ferrous-bearing materials, like fasteners, brackets, flashing and railing, which are rusting onto the EIFS.

Robert Thomas is a nationally recognized EIFS consultant, based in Jacksonville, Fla.
He was the manager of technical services for a major EIFS producer, is the
author of several books about EIFS and chairs the ASTM committee on EIFS.

Check out the May 2020 edition of Walls & Ceilings: The devoted team of Green Country Drywall at a recent job site in Oklahoma, rainscreen drainage and ventilation mats, SBA loans via the CARES act, and much more!